1. Field of the Invention
The present invention relates to a structure of a back light unit arranged at rear surface of a back light type liquid crystal display device and a method for manufacturing the unit.
2. Description of the Related Art
Generally, a back light type liquid crystal display device is constructed so that a light guide plate made of transparent synthetic resin such as acryl is provided at rear side thereof and light from a light source arranged at one end surface or both end surfaces of the light guide plate is scattered to whole liquid crystal display device through the light guide plate. A reflective sheet is laminated at a rear surface opposite to the liquid crystal display device among the light guide plate, on the other hand, at a surface of the liquid crystal display device among the light guide plate, a brightness rising sheet is laminated at least one light diffusion sheet made of synthetic resin, or the light diffusion sheets are laminated in layers as the need arises.
In this case, at least one light diffusion sheet, the light diffusion sheet and the brightness rising sheet, or the reflective sheet laminated on the light guide plate is fixed on the light guide plate.
For fixing, there are a method using adhesive double-coated tape or stickness adhesive and a method welding. The former method using adhesive double-coated tape or stickness adhesive incurs remarkable cost-up because of manual work, and in the later method welding, welding area is small when each sheet is made of synthetic resin material different in kind of the light guide plate, thereby the method can not be applied because of low welding strength.
Then, Japanese Patent Laid-Open Hei 7-043713 of the related art proposes to drive a metal sticking member such as eyelet into the light guide plate so as to reach the light guide plate passing through the various kind of sheets by ultrasonic vibration as the method fixing light diffusion sheet, the light diffusion sheet and the brightness rising sheet, or the reflective sheet.
However, the method by the related art makes not only the cost for fixing but also cost for the production of the back light unit increase considerably because the metal sticking member such as eyelet is provided particularly and the metal sticking member is driven into the light guide plate. Further, there is a problem that thickness of the back light unit increases, the size becomes large, and the weight remarkably increase because a head portion of the driven sticking member projecting from the surface of the sheet.